Effect of process parameters and the capacity of the microemulsion reactor
In order to achieve the maximum yield of phenolic monomers and representative products, the process parameters such as reaction temperature, time and the amount of catalyst were optimized by using the microemulsion system located at point c (Figure 3). Results (Figure S4) show that the reaction temperature and time have similar effect and tendency on the oxidation of lignin, the maximum yield is obtained at 433 K for 4 h, achieving 119.9 mg g-1 of phenolic monomers and generating two typical value added chemicals, i.e. ,p- hydroxy benzaldehyde and propyl-4-hydroxybenzoate with the yield of 48.2 and 21.2 mg g-1 respectively. In addition, the dosages of CuSO4 and H2SO4 for lignin oxidation in this system were also investigated (Figure S5) and the optimized dosages of CuSO4 and H2SO4 are 0.1 mmol and 25.0 mmol L-1 respectively. It is interesting to note that the changing trends of p -hydroxy benzaldehyde and propyl-4-hydroxybenzoate in Figure S5b are opposite, it suggests that the excess of H2SO4 can promote the oxidation of p -hydroxy benzaldehyde to p -hydroxybenzoic acid, which can be esterified easily with n -propanol under the acidic condition.
In addition, the oxidation performance of this microemulsion system for technical and organosolv lignins from different resources was studied under the optimized conditions and results are summarized in Table S2. It can be found that this microemulsion system is also appropriate to depolymerize other organosolv lignins into aromatic compounds. 101.2 and 109.1 mg g-1 of phenolic monomers can be produced when bamboo and miscanthus organosolv lignins are used as the substrates respectively, while it provides 84.8 mg g-1 yield over poplar organosolv lignin. It implies that herbaceous lignins are much easier to be depolymerized than hard wood lignins, due to less H unit containing in hard wood lignins,45 which accords well with the above results about the selective tailoring of H unit. Besides, the yields of phenolic monomers from technical lignins are lower than those of organosolv lignins, in line with the more plentiful C-C bond containing structure of technical lignins.5Quantitative analysis of volatile chemicals derived from bagasse organosolv lignin oxidation under the optimized conditions is exhibited in Figure S6 and Table S3. There are 12.7 wt. % of phenolic products, including 52.4, 44.8 and 2.8% of H, G and S unit products. From this perspective, it can imply that H unit in lignin can be depolymerized easily, followed by the order of H > G > S unit in this case.